Haile Gebrselassie
Not just distance running, not just track, ALL sports. All athletes in human history.
Find another athlete who has his combination of dominance, longevity, consistency, durability, achievement, range, degree of difficulty of sport, and competition depth of sport, gap over rivals/competitors in sport, and then put him or her up there, and debate it. You may find one who can compare on some, or several, of those categories, but not all.
28 WR's at every conceivable (non-steeple) distance ranging from 2K to marathon (No other runner is anywhere near that)
Broke at least 1 WR in 13 different years, over a succession of 16 years
19 consecutive years ranked at or near the top in at least one event
Highest marathon winning percentage in history (10 race minimum)
Highest margin of WR gap average in history (By far)
Never missed a season, or significant amount of time, due to injury in one of the most unforgiving and grueling sports
So, take your stab at it. I don't think there is anyone that can compare. You look at someone like Michael Phelps, who is certainly in the discussion, and can match Geb in many categories, but he doesn't have the longevity, competition depth of sport. Or degree of difficulty of sport. (Swimming is tough, but not on your joints and bones). Pele? Did he have that range? Gap on his competitors? Nadia Comaneci? Longevity in gymnastics is relative, but she had only a 5 year span. Those are a few, there are others, but who can match Geb?